Tharyx hessleri Blake 2019, new species

Tharyx hessleri new species

Figures 19–20

urn:lsid:zoobank.org:act: 7472BB3A-8A19-42D9-B2EF-114C06DA01B7

Chaetozone sp. A: Wilson & Hessler 1987: 66 Appendix E (in part).

Material examined. North Equatorial Pacific Ocean, abyssal plain, Clarion-Clipperton Fracture Zone, ECHO I, DOMES Site C, R/ V Melville cruise, coll. R. Hessler, Sta. H 350C, 0–1 cm fraction, 4 Jun 1983, 14°38.1226ʹN, 125°26.8208ʹW, 4506 m, holotype (LACM-AHF Poly 11287); Sta. H 360, nodule wash, 17 Jun 1983, 14°40.7987ʹN, 125°22.0379ʹW, 4500 m, paratype (LACM-AHF Poly 11288).

Description. A small, threadlike species; holotype complete, 3.72 mm long, peristomium 98 µm wide; anterior, middle, and posterior setigers of same width, about 92 µm, last few setigers narrowing slightly; with 39 setigers. Anterior setigers about 1.5 times as wide as long (Fig. 19A); middle and posterior segments becoming somewhat rounded, as long as wide, with far posterior setigers weakly moniliform (Figs. 19B, 20 B–C). Paratype (LACM-AHF Poly H360) smaller, broken in two parts, only 2.0 mm long with 20 setigers. Body cylindrical in cross section, with no dorsal or ventral grooves. Color in alcohol opaque white; no pigment.

Pre-setiger region approximately twice as long as wide, slightly longer than first three setigers (Figs. 19A, 20A). Prostomium triangular, tapering to narrow apex (Figs. 19A, 20A); eyespots absent; nuchal organs not observed. Peristomium entire, smooth, without annular rings (Fig. 19A); dorsal tentacles arising from posterior margin; first pair of branchiae arising immediately posterior and slightly lateral to dorsal tentacles (Fig. 19A). Second pair of branchiae arising from setiger 1, dorsal to notosetae; subsequent branchiae from similar location. Branchiae present as stubs over anterior and some middle segments, not observed on posterior segments.

Parapodia reduced to low mounds from which setae arise. Setae of anterior segments all capillaries with 4–5 notosetae, including some long, natatory-like, continuing through middle body (Figs. 19A, 20A); capillaries of middle and posterior segments shorter, 4–5 per fascicle. Holotype with neuro-acicular spines first present from setiger 18 in neuropodia and far posterior setiger 32 in notopodia. Neuropodial spines typically numbering 1–2 at first, accompanied by 2–3 capillaries; in posterior segments notopodia with 1–2 spines and 1–2 capillaries, neuropodia with 2–3 spines and capillaries present or absent. Notopodial spines of far posterior segments longer than those in neuropodia, but both sets of spines emerging prominently from posteriormost setigers (Figs. 19B, 20 B–C). Spines curved, geniculate, terminating in stubby knob-tipped apex (Fig. 19 C–D); no denticles observed along shaft.

Pygidium a thick rounded, ventrally directed lobe, surface with a coarse granular texture (Figs. 19B, 20C).

Methyl Green stain. No pattern, de-stains entirely.

Etymology. This species is named for Dr. Robert E. Hessler, prominent deep-sea ecologist whose pioneering work on deep-sea benthos, including collection of the ECHO I samples reported in this study, has inspired generations of scientists.

Remarks. Among the 15 described species of Tharyx, only two species have been recorded from depths of 2000 m or greater: T. kirkegaardi Blake, 1991 from off the U.S. Atlantic coast in slope and abyssal depths to 3000 m and T. moniliformis from the Weddell Sea in 2086 m. Tharyx hessleri n. sp. from 4500 m is therefore the deepest recorded for any species of the genus. Both T. kirkegaardi and T. moniliformis have serrations along the shaft and prominent knobs on the tips of the acicular spines thus providing a sub-bidentate appearance; in contrast, the knobs on the spines of T. hessleri n. sp. are rounded off, providing a worn appearance and there are no serrations along the shaft.

Distribution. Abyssal Pacific Ocean, 4500–4506 m.